In lighting systems and in society as a whole, there is a trend towards sustainable solutions. One way to achieve more sustainable systems is to reduce energy consumption. In lighting systems this is often done by making use of sensors. One possibility is to make use of presence sensors, so as only to switch on the lights when a person is present. Another possibility is to make use of daylight harvesting. When enough light is available in a room from daylight, the lights inside are dimmed to reduce the energy consumption. It is also possible to implement systems that include both presence detection and daylight harvesting.
One issue with daylight harvesting systems is the need for calibration. For example in daylight harvesting systems, a light sensor may be located on the ceiling. This light sensor measures an overall light level which comprises both light from the window and light from the electrical lights inside, e.g. as reflected from a work surface such as a desk. A control loop varies the amount of artificial light emitted so that the total sensor reading meets a desired set-point, to thereby keep the overall amount of light at the desired level. This means a calibrated set-point will have to be defined, i.e. to define what sensor reading corresponds to the desired light level. Calibration may also be an issue in other situations.
In current practices, calibration is performed by a commissioning engineer using another, already-calibrated light sensor. He or she controls the amount of light and by dimming up and down, and makes sure that typical 500 lux fall on the desk. This calibration step is time consuming and therefore expensive.
U.S. Pat. No. 7,781,713 (Papamichael et al) discloses an automatic calibration process, in which calibration is performed by controlling the light output to automatically cycle through different energy levels, and measuring the difference in the light sensor measurement experienced under the different levels.